The present disclosure relates to magnetic resonance imaging More particularly, the disclosure relates to a system and method for selective imaging of arteries or veins using magnetic resonance imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Magnetic resonance angiography (MRA) and, related imaging techniques, such as perfusion imaging, use the NMR phenomenon to produce images of the human vasculature or physiological performance related to the human vasculature. There are three main categories of techniques for achieving the desired contrast for the purpose of MR angiography. The first general category is typically referred to as contrast enhanced (CE) MRA. The second general category is phase contrast (PC) MRA. The third general category is time-of-flight (TOF) or tagging-based MRA.
Contrast-enhanced MRA (CEMRA) is frequently used to evaluate vascular disease. Although arteries are well evaluated using CEMRA, it is problematic to show veins without troublesome overlap from arteries. Moreover, high contrast doses or a costly blood pool agent may be required. Furthermore, these techniques require the use of exogenous contrast material. Such agents are costly and expose the patient to added safety risks, namely, nephrogenic systemic fibrosis. As such, non-enhanced MRA (NEMRA) techniques are helpful for the evaluation of suspected vascular disease in patients with impaired renal function, since they avoid the risk of nephrogenic systemic fibrosis.
Examples of newer non-enhanced techniques include quiescent-inflow single-shot (QISS) MRA, fresh blood imaging, and flow-sensitive dephasing, such as described in co-pending U.S. application Ser. No. 12/574,856, which is incorporated herein by reference in its entirety. QISS MRA has been shown to be a fast, accurate method for non-contrast MRA. However, imaging of veins can be problematic using traditional QISS techniques due to the intermittent nature of venous flow. In certain regions like the lower-extremities, venous flow may be entirely absent for extended periods of time, so that flow-dependent MRA techniques like QISS do not reliably show venous anatomy.
Additionally, flow-independent MRA techniques are potentially advantageous because they can be implemented as a 3D acquisition with excellent signal-to-noise ratio, high spatial resolution, and insensitivity to abnormal flow patterns. Flow-independent MRA techniques can be performed during the steady-state, after administration of a blood pool contrast agent, such as gadofosveset trisodium, or can be acquired without contrast agents using pulse sequences such as 3D balanced steady-state free precession. However, the projective images overlap between arteries and veins severely limits the diagnostic utility of images acquired using flow-independent MRA. Attempts have been made to suppress venous or arterial signal by applying a saturation pulse outside of the imaging region. However, the saturation pulse is ineffective because, unlike the case with thin, 2D slices, inflowing saturated blood undergoes T1 relaxation and substantially recovers its signal intensity before it penetrates far into a large, 3D volume.
Thus, it would be desirable to have a system and method for non-contrast-enhanced imaging arteries and veins and/or flow-independent MRA imaging techniques that do not suffer from the challenges set forth above and other challenges.